Presently, the anti-corrosion property of a magnet of a hard disk driver (HDD) spindle motor, a laminated core, or other parts is mainly realized through electrodepositing coating over the surface. The associated mechanism is to make use of the shielding function of coating to isolate the base from the surrounding environment so as to prevent the metal base covered by the coating layer from corrosion that would otherwise occur due to contacting with outside corrosive medium.
Generally, when a motor which is made of spindle motor parts with an electrodepositing coating layer is used in a hard disk driver, the storage content in the hard disk tends to lose, which is caused by adherence of tin and its compound released from the electrodepositing coating layer to the surface of the magnetic storage medium. Therefore tin content in the coating layers is conventionally regulated to a quite low level. For example, the patent No. CN1073754C, titled A Magnet and a motor part with an anti-corrosion coating layer and increased insulation, regulates tin content to a level of 50 ppm or even lower, i.e., the content of tin and its compound in the aqueous electrodepositing paint solution is equal to or lower than 12 ppm to the effect that the magnetic storage medium would not be damaged.
However, it is simply not easy in the electrodepositing process to regulate the tin content in the aqueous electrodepositing paint solution to the level of 12 ppm or even lower since tin and its compound may be contained in impurities of raw materials. In other words, it is very difficult to carry out such a process.
Tin presented in the electrodepositing coating layer takes both forms of organotin and inorganic tin. Our studies with respect to this reveal that data loss of a hard disk can be only attributed to organotin in the electrodepositing coating layer, and inorganic tin would not cause data loss. Therefore, it suffices to regulate the organotin content to a level of 50 ppm or even lower, thus rendering the electrodepositing coating process undemanding.
The method of coating a single electrodepositing coating layer over the spindle motor part of a HDD as per the patent No. CN1073754C is defective in that those sections of the part base that touch the electrode needles turn into pinholes, not covered by the electrodepositing coating layer and therefore exposed to the surrounding environment. Thus exposed base is directly subject to the environment, leading to corrosion and hence potential damages to the HDD.